PHYSICAL FACTOR RESULTS 97. 



exclusion of single curves. Combinations are made simply by tracing the 

 curves to be compared upon the same sheet, it being understood that dis-. 

 similar curves, e. g., level and station, can not be combined. Colored inks 

 are an absolute necessity in combining; the primary principle underlying 

 their use is that curves that approach closely or cross each other must be 

 traced in inks that contrast sharply. As elsewhere stated, it has been made 

 the invariable rule to use the same color for the same level or point. This 

 applies especially to temperature, but holds also for humidity, light, wind, 

 and water-content, so that the color always indicates the level. For the 

 same reason, it is applied to a combination of point curves for one station, 

 though it is inapplicable to a series of point curves when these lie in the 

 same level. Light readings above 6 feet and water-content readings below 

 15 inches necessitate the use of additional colors. 



Combinations may be made of the curves of a single factor for purposes 

 of comparison, or they may consist- of curves of different factors in order 

 to aid in interpreting or indicating their relation to each other. Curves of 

 the same factor may be combined to form various series. The level series 

 consists of all the level curves for the stations under observation, e. g., 

 the six levels for temperature, three levels for wind, etc. Similarly, the 

 station series is a combination of all the station curves, and a correspond- 

 ing arrangement may be made for point curves with reference either to 

 station or to level. An extremely valuable combination of curves is that of 

 the holard and chresard for a series of stations. The most important com- 

 binations of the curves of different factors are naturally those based upon 

 factors intimately related to each other or to the plant. The grouping of 

 water-content and humidity curves is of great value, especially when the 

 transpiration curve is added. Light and temperature curves make an in- 

 teresting combination, while a humidity, temperature, and wind series is of 

 much aid in tracing the connection between these factors. Finally, it is 

 altogether feasible to arrange the curves of water-content, humidity, light, 

 temperature, and wind upon the same sheet in such fashion as to give a 

 graphic representation of the whole physical nature of a single habitat or 

 a series. In all combinations of curves representing dift'erent factors, it 

 must be borne in mind that the position of a curve does not represent a 

 definite value with reference to the others, since some are based upon per 

 cents, others upon degrees, etc. The comparison must be based upon the 

 character of the curves, but even then it is an important aid. An instruc- 

 tive grouping has been employed where series of readings on the same day, 

 or on two successive days in forest and in prairie have yielded the usual 

 level series of curves. The series for the two habitats are arranged on 

 the same page, one at the right and the other at the left, and permit direct 



